1. Field of the Invention
The present invention relates to a method and an apparatus for generating dot signals to record halftone images of four color separations of Y (yellow), M (magenta), C (cyan), and K (black), and more specifically to a technique to adjust screen angles of the four color separations for each image area included in a total image.
2. Description of the Related Art
A color print is usually reproduced by overprinting halftone images of color separations for four process colors, that is, Y (yellow), M (magenta), C (cyan), and K (black). A halftone image of each color is a collection of dots whose size is determined according to the density of each color. In a halftone image, an orientation of dots with respect to a predetermined reference direction (for example, a scanning direction of a recording scanner) is called "screen angle". As is well known in the art, screen angles .theta.(Y), .theta.(M), .theta.(C), and .theta.(K) of the four color separations usually have different values, for example, .theta.(y)=0.degree., .theta.(M)=45.degree., .theta.(C)=15.degree., and .theta.(K)=75.degree., in order to prevent a moire which might be generated by overprinting halftone images of Y, M, C, and K.
Generally, the yellow separation has little effect on generating a moire because density of yellow separation is lower than the other color separations. However, a moire is sometimes observed in a thin green-colored image area which is mostly painted with yellow and cyan inks. A moire is also observed occasionally in a skin-colored image area which is mostly painted with yellow and magenta inks. The moire related to yellow can be distinctively observed when the muddiness in yellow ink is high. Elimination of the moire is therefore highly required even in those image areas which are painted with two color inks including yellow.
All the four color separations of Y, M, C, and K are not always used for printing each image area within a total color image and in many cases, a linework image is printed only in two or three color inks. The moire should also be eliminated in such image areas in the total image.